Tortillas are a popular food and have been made and served fresh for many years. Because they are made and served fresh they require a shelf life of only two or three days. This is adequate for restaurant uses and home use when made from scratch but has not been adequate for consumer sales because of degradation over the shelf life required for typical consumer sales distribution systems.
There has been an increase in the popularity of Mexican foods in recent years. However, the ability to distribute tortillas through normal channels of sale and distribution has limited their commercial production and acceptance. Some success has been achieved to date by varying formulae and processing conditions to achieve improved distribution and product quality. However, the maximum shelf life obtainable for these products, whether they are shelf-stable or refrigerated, has been limited both in duration or quality at the time of consumption. A typical, good shelf life of current products on the market is, at best, about ten (10) days.
The main mode of failure of these products is loss of flexibility. With a loss of flexibility when the tortilla is filled and wrapped, the shell breaks along the folds causing loss of contents and loss of appeal. Even with reheating of current product, as for example on a griddle, there is some regaining of flexibility but not sufficiently to have a high-quality product.
It is believed that the loss of flexibility is due to staling of the product, which is probably a retrogradation of the starch. Staling is a very complicated and little understood phenomena, and solutions to staling and/or retrogradation generally have not been successfully achieved. However, one recent solution to staling has been provided and is disclosed in U.S. Pat. No. 4,456,625, Storage Stable, Ready to Eat Baked Goods. This reference retards staling by substantially preventing gelatinization of the starch during cooking. Such a solution was adequate for some products but was not amenable to tortillas because tortillas need to be highly gelatinized for strength.
The starch in baked goods is by no means a system in thermodynamic equilibrium. Changes in the starch gel structure referred to as staling start immediately after cooling. Staling is a complex physiochemical phenomena and a considerable economic problem encountered in the baking industry. It refers to total family of post-production changes that occurs during the distribution and shelf life of a starch based product. Numerous studies to determine the cause of bread staling have indicated that changes in the starch component are of major importance. The predominant mechanism of staling is the recrystallization of starch from the amorphous state of the freshly heated product to the partially crystalline state of the stale product, with concomitant redistribution of moisture, increased firmness, changes in mouthfeel, and changes in flavor. The recrystallization depends strongly on the history of the product; both the initial heating and the type of ingredients will contribute to this overall retrogradation process. However, as will be described later, the starch in the present invention is cooked to substantial gelatinization.
Due to confusion associated with the definition of gelatinization, a brief explanation of this phenomena is provided.
The starch granule is a partially crystalline glassy polymer system composed of two molecular entities: amylopectin which is a glucose copolymer and amylose which is a linear homopolymer of glucose. Gelatinization refers to the sequence of changes which occur when starch is heated with water. During this heating process, the appearance of granules does not change until a certain critical temperature range is reached, at which the granules swell and lose their polarization or birefringence cross. Both swelling and loss of birefringence start at the botanical center, or hilum of the starch granule, and rapidly progress to the periphery. The loss of birefringence is a simple and widely used test as an indication of starch gelatinization. However, it only indicates a degree of gelatinization. The difference in the degree of association in the amorphous regions of individual granules is responsible for gelatinization occurring over a temperature range rather than at a single temperature.
After loss of birefringence, the granules continue to swell, and solubles leach from the granules into surrounding media i.e. become further gelatinized. The diffusion of solubles, primarily amylose from the granules during the gelatinization, and swelling and partial distortion of granules make contact between the granules possible thereby forming a continuous matrix. This continuous starch gel in conjunction with a gluten matrix, when gluten is present, will provide the structural strength to the product.
It was found that to achieve adequate structural strength in the tortillas the internal starch granules are substantially gelatinized, as measured by loss of birefringence. An underbaked product failed to perform because of lack of structural strength. A strong correlation was found between the degree of starch gelatinization and product performance (flexibility) over time.
Tortillas, as are known in the art, normally contain high moisture and are high A.sub.w (water activity level) products. Such a high water formula aggravates the staling or retrogradation phenomena, particularly at refrigerated temperature storage. Further, if such a product is not stored under refrigerated or frozen condition, then preservatives are required to prevent microbial degradation of the product.
The desire to not use preservatives in tortillas complicates the solution to staling because in order to not use preservatives, refrigerated storage, a traditional way of storing tortillas, aggravates the staling phenomena. It is known that refrigerated temperatures about 4.5.degree. C. (40.degree. F.) cause the fastest staling rate in starch-based products. Thus, the use of a traditional refrigerated distribution system results in the inherent loss of flexibility in the product at the fastest rate.
Tortillas currently made are a mixture of flour, water, salt and oil to form a dough which is then sheeted and cut into the tortilla size and shape. The cut dough pieces are then baked, packaged, and stored. An evaluation of such commercially produced products shows that they are cooked very little, just enough to provide structure to the product to ease its handling. An evaluation of such products indicates that the starch is not completely gelatinized as measured by a loss of birefringence and by its Water Holding Capacity (WHC).
The problem of how to provide a tortilla which has good flexibility and long shelf life seems to be mutually exclusive when refrigerated storage is to be used. An object of the present invention is to provide an improved tortilla still that is traditional in nature providing the same eating, taste and use characteristics as a traditional tortilla in order to maximize consumer acceptance of the product as a tortilla. Another object of the present invention is to provide a tortilla which has improved shelf life with good eating characteristics, and be convenient and easy to use.